A Review on Recent Studies: Non-Traditional Machining of Titanium Alloys

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Introduction
Unique material properties makes titanium alloys to a widely used alloys in the automotive, aerospace, petroleum, chemical, marine and biomedical industries. It's commonly used in aeronautic industry due to the characteristics like low density, strength and high temperature resistance. However, it also considered as an expensive material because of deformation and fabrication process. In many cases, it's required machining works where titanium alloys categorized as difficult-to-cut materials. There are several materials, mechanical and machining properties that influence to the titanium alloys machining process Gupta and Laubscher ( ); Niknam et al. ( ); RAHMAN et al. ( ) such as low thermal conductivity which leads heat concentration on the cutting edge; high chemical reactivity causes rapid wear, chipping or cutting tool failure; continuously high temperature and stress generation in between chip and cutting edge (within . mm); low elastic modulus. To solve these problems, research studies is being carried out into new tool developments considering tool design and tool materials; optimization of cutting parameters(cutting force, speed, depth of cut, feed rate etc.) Niknam  A number of advanced machining process performance investigation in hard-to-machine materials including titanium alloys machining has taken part of the research concern due to the capability for overcoming existing problems. The shows the research works on non-traditional machining of titanium alloys. In this case, ultrasonic machining, electrochemical machining (ECM), electrical discharge machining, laser assisted machining, abrasive water jet machining (AWJM) are considered as non-traditional machining process. The graph represents the number of research articles has published in major journals in the year to present. It is clearly indicates that increasing of research in the field of titanium machining. The growth in USM, ECM and EDM significantly higher than LAM and AWJM. However, number of research works is uplifting for the LAM and AWJM. In addition, research issues in non-traditional machining processes can be seen as machinability in term of comparison between conventional and non-traditional machining; modelling and optimization; influence of process parameters; performance such as surface quality after machining process completed, temperature during machining process etc. (see ). For machining process like USM, Figure : Number of articles on non-traditional machining of titanium alloys extracted from different journals database.
ECM and LAM, data shows that research works are carried out to investigate influence of process parameters and machining performance such as surface quality of machined workpiece much more than the investigation of machinability and modelling and optimization. Moreover, for EDM the graph indicates the highest research works done in machining performance. For AWJM, surprisingly research on modelling and optimization has taken place the top among the others. Overall, the graph reveals the research focus in non-traditional machining process.
Figure : Percentages of studies categorized by types of non-traditional machining extracted and summarized from different journals database.
There are several research works has been done in every year that shows research direction or trends. However, most of them are specific in one machining process. Sometime that create complexities for new research direction. In addition, it's important to have a clear picture of a research area for further research consideration. Therefore, this study aim to review last ten years articles in non-traditional machining process of titanium and its alloys. The focus of this review will be on the contribution for solving existing problems by using non-traditional machining processes mentioned earlier. This article organized with the start of introduction, secondly, recent works of non-traditional machining and thirdly, improvement compared to conventional machining process. Then, finally concluded with the suggestions of most efficient machining process for titanium alloys machining. In this study, most of the articles are collected from major journals.

Ultrasonic Machining (USM)
Ultrasonic machining or ultrasonic vibration assisted machining process remove materials from workpiece through high frequency and low amplitude vibrations, piezoelectric actuator used as the source of vibration. The frequency is about to kHz. This machining process used as an alternative option of EDM and good choice for hard materials machining. USM considered as one of the high efficient machining process for titanium alloysXu and Zhang ( ). Result of Maurotto et al. ( ) shown that at least % of cutting can be reduced compare to conventional machining of titanium alloys while using USM. He also provides a comparison of surface quality which revealed a clear statement on advantage of USM in term of surface quality compare to conventional machining. In the study of Koshimizu ( ), shows a reduction of -% cutting force. At the same time, the study states that semi-dry cutting condition is suitable for ultrasonic assisted turning. Similar improvement results found in different studies using USM in term of surface quality, cutting force and cutting temperature Maurotto et al.  It can be seen from the previous section that most of the research output concluded about the comparison and improvement. Among them, surface quality, cutting force, cutting temperature was the main concern in their studies. However, some other issues such as influence of cutting fluid, ultrasonic parameters also has been considered. Dixit et al. ( ) concluded in his review article that it is important to have research focus on influence of friction in ultrasonic vibration assisted machining process. He also found less studies from economic and power consumption perspectives. So, the further study can be to emphasize on sustainable manufacturing for titanium alloys machining process.

Electrochemical Machining (ECM)
The electrochemical machining (ECM) is a modern nontraditional method of machining based on the electrochemical dissolution of metal aimed at the production of parts of prescribed shape, dimensions, and surface finish McGeough ( ); Rumyantsev and Davydov ( ); Wilson ( ). There are several application of it in titanium part preparations such as gas turbine engine parts, engine blades etc. Figure gives a scheme of one type of ECM, the formation of a hole of circular cross-section using a tubular toolelectrode (TE). A hole is formed by deepening TE into the workpiece at a constant rate equal to the rate of metal dissolution at the bottom of the hole. Thereby, the inter electrode distance (the machining gap SF) is held constant. To enhance the accuracy of replication of TE onto the workpiece (WP), the inter electrode distance should be as small as possible, frequently, less than µm. To remove the products of electrochemical and possible chemical reactions from the working zone and refresh the electrolyte composition in the working zone, an intense electrolyte flow is organized.
Many specific features of ECM depend on the type of workpiece, type of required operation, and the technology applied. For example, the machining of large-sized parts and complex-shaped parts on special machines, the machining with the removal of large amounts of metal and application of powerful sources and large volumes of electrolyte differ significantly from the micro-and nanomachining with simple-shaped TE in the form of a needle or plate on small machines with the dissolution of small amounts of metal Davydov et al. ( ). In addition, ECM does not cause heat-affected zones and internal stresses on the machining surface Xu et al. ( ), therefore it can produced parts with high accuracy and better surface quality for hard-to-cut material like titanium. There are different types of ECM, however, micro-ECM is become very popular in recent years due to the high accuracy, small amount of material removal with a low current which better than conventional ECM. Unfortunately, this method can't be applied for the titanium as highly prone to passivation metal when coated with thick passive resistive oxide films, because the oxide films exhibit significantly higher resistance than the solution layer in the inter electrode gap. In addition, it is important to choose an appropriate electrolyte and relevant current parameters. Xu et al. ( ) concerned that self-passivation is one of the problem in ECM process, which inhibits the dissolution process. Therefore, he suggested to select corresponding process parameters. Influence of process parameters Anasane and Bhattacharyya ( ) is given below: . Electrolyte: Electrolyte plays an important role in term of surface quality, texture, accuracy, material removal rate etc. for example methanol electrolyte is better for shape and surface; Solution of methanol sulfuric acid has effective in electro polishing of titanium.
. Machining Voltage: voltage between cathode and anode is also critical for controlling anodic dissolution. Different amount of voltage has influence to remove material in different rate.
. Pulse Duty Ratio: Duty ratio represents the percentage of time for which pulse remains on i.e. percentage of time available for both faradic and non-faradic current. Increase in duty ratio in turn increases time available for faradic current. Hence, the amount of faradic effect and current density increases results in more material removal.

Electric Discharge Machining (EDM)
Electric discharge machining (EDM) is one of non-traditional material removing process, which is successfully used in difficult-to-machine materials like titanium. Many industry used the process due to the high accuracy, ease of cutting complex shape and better surface roughness. EDM process introduce a residual stress free material because of the tool (electrode) and workpiece material untouched condition Qudeiri et al. ( ). The principle of the EDM technique is to use thermo-electric energy to erode a workpiece by rapidly recurring electrical discharges (sparks) between the untouched electrode and workpiece. The small distance between tool (electrode) and workpiece named discharge gap, which filled with dielectric fluid. Flushing the dielectric fluid during the machining process carries away debris (removed solid particles) and restores the sparking condition in the gap. As no cutting forces are used in between tool and the workpiece results an untouched condition which helps to eliminates vibration or stresses during machining.

Figure : Classification of Electric Discharge Machining Process
In EDM process there are several process parameters such as peak current, pulse off time, discharge voltage, pulse on time, polarity, pulse wave form, discharge gap, flushing of the dielectric fluid, and workpiece rotation; which are affected the EDM performance e.g., surface quality, material removal rate and electrode wear rate(EWR). MRR is one of the concerning issue in EDM due to the low material removal rate compare with the other non-traditional machining process Qudeiri et al. ( ). EDM forms in different name with basic principles in various process. Qudeiri et al. classified EDM process in category shown in figure . Many research has been carried out to investigate the effect of process parameters on performance of titanium alloys in EDM process. Moreover, several methods has been introduced to improve the performance such as surface quality, machining accuracy and material removal rate. Bhaumik and Maity investigated effect of electrode material in different EDM aspect of titanium alloys. They concluded that copper and brass electrode provides good surface finish Bhaumik and Maity ( ). In addition, zing electrode resulted higher defects. Some of recent studies of titanium alloys in EDM process is shown in the Table . In the EDM process, several research objective reported in recent works namely modelling and optimization; surface quality; dielectric fluid research; electrode materials used in EDM process; Machinability and process performance.  Different powders such as aluminum, chrome, silicon carbide, graphite and iron is performed to investigate the output characteristics of surface roughness and topography, material removal rate (MRR), electrochemical corrosion resistance of machined samples and also the machined surfaces are investigated by means of EDS and XRD analyses Table : Studies of titanium alloys in EDM process.

Laser Assisted Machining (LAM)
The idea of laser assisted machining (LAM) has come with the concept of localized heat for reducing cutting force during machining process, and ultimately it results decrease in yield strength Dandekar et al. ( ). In this process, workpiece material heated by laser (e.g., CO and Nd:YAG) and material removed with traditional cutting tools Venkatesan et al. ( ). For titanium alloys treatment Nd:YAG and CO usually used, characteristics of this laser type shown in table . Most of the research is concerning advantages of LAM and challenges in traditional machining process. However, the performance of LAM depends on various parameters such as laser parameters and machining process parameters. Laser power and spot diameter of laser beam are main parameters of LAM. In term of influence of process parameters and it performance, many studies has been conducted. As a hybrid machining process, investigation of LAM provides various results. Table , shows a summary of LAM machining process and performance of titanium and its alloys. Cutting force reduced by -%; Cutting speed can be increased % without reducing tool life; Compressive residual stress reduced by %; A % cost can be reduced using LAM Table : Studies of LAM on titanium alloys.
Among the types of machining process, some important issues, challenges and opportunities exist. For hybrid conventional laser assisted machining process it can be categorized by machining cost, tool life, and flow stress. At the same time the challenges are to overcome by solving the problem of complex control system and application to material with various shapes.

Abrasive Water Jet Machining (AWJM)
Abrasive water jet machining (AWJM) is one of the flexible material removal method that become popular among other machining process. The reason behind this is elimination of thermal effect and minimum stress in workpiece Kartal ( ). In this machining process, mechanical energy of water with the abrasive particle used to cut the material by erosion Li and Wang ( ). The water pumped to high pressure ( -MPa) and exit through the nozzle (ranging from . to . mm) Shukla ( ). The velocity of water is about m/s which used to cut materials like metals, ceramics, glass etc. To improve the machining performance for abrasive water jet process, many research works has been conducted. As hard-to-cut material and thermal characteristics of titanium alloys, number of research has been done. Most of the research focus on the influence of machining process parameters, machining performance such as surface quality and the modelling and simulations. Table , shows the summary of the recent work on titanium and its alloys using abrasive water jet machining.
Author ( The most significant factors for the taper angle on both materials are the pressure and the stack configuration, followed by the traverse feed rate. To minimize the taper angle and the roughness, it is recommended to use high-pressure, low traverse feed rates. Li and Wang ( )

Water pressure drilling time
Effect of process parameters It was found that both the hole depth and diameter increased as drilling time increased but in a decreasing rate. An increase in water pressure increased both the hole depth and the hole diameter. Some other researchers also focus on surface quality and classifying the surface of workpiece material. Pal and Choudhury ( ) generated pocket on titanium surface with adjusting parameters namely pressure, standoff distance and abrasive size. Surface waviness, roughness and crater has been measured. Results shown that small abrasive size provides good surface quality. In another work, Kerf geometry, surface roughness (Ra), microstructural analyses and material removal rate (MRR) of Ti-Al-v were investigated using Abrasive water jet machining Gnanavelbabu et al. ( ). He mentioned surface roughness is affected by high pressure of water jet. Results of Babu and Muthukrishnan ( ) also provides conclusion for surface quality.

Sustainable issues in Non-Traditional Machining
Sustainability is one of the important issue in modern machining process. For some complex production, it is impossible to produce the product without non-traditional machining process. The concerning issues can be affected health, environment or even economy.

Conclusion
Non-traditional machining process for titanium alloys due its capability to solve existing problems. Research works in the field of non-traditional machining is still going on. This paper reviewed several articles on different types of non-traditional machining process. Therefore, this section can be arranged with some conclusions.
. First section of this paper presented current trends on non-traditional machining process. Its shows EDM has the first priority for the research work as the number of published articles in last years. Moreover, the research focuses specifically on investigation of machining process parameter and performance.
. Ultrasonic machining has successfully reduced cutting force, temperature and improve surface quality for titanium alloys. However, more research need to have in term of cutting mechanics and solving sustainable issues such as energy consumption and economic aspects.
. For electric discharge machining, review shows that dielectric fluid has significant effect on machining performance. In addition, low discharge energy provides better surface with less defects. Integration of other hybrid process resulted better outcome. Future research can be focused on combining EDM with other process like laser assisted machining.
. As the main problem of electrochemical machining on titanium is prone to passivation in the aqueous electrolyte solutions. Which is required for high amount of dissolution of titanium. Therefore, electrolyte solutions can be the focused for the further research. In addition it's important to have focus for sustainable issue as ECM machining process used much more chemical compared to other non-traditional machining process.
. Laser assisted machining has already achieve great attention for the capability of overcome by reducing cutting energy and improve surface roughness of titanium alloys. The review reveals that LAM significantly improve the product quality by decreasing cost.
. In Abrasive water jet machining Garnet, silicon carbide dust, and aluminum oxide are most commonly used as abrasive materials, however more research work need to do for further development of this process.
. Sustainability is most common issue in modern manufacturing process. Specially, for non-traditional machining process, there are not many studies has been carried out. Therefore, more research should be focused on environmental and economic issues.